For two lists [e1,e2,e3] and [f1,f2,f3], e1 is compared to f1. The lower (dictated by the standard ordering below) is put into List3, and the process continued with the remaining input lists. This process continues until both lists are exhausted.
In particular, this will merge two sorted lists into a sorted list.
The sort is done according to the standard ordering of terms. Duplicates are not removed. See compare/3 for this standard ordering. Note in particular that numbers are first ordered by their type (integer, float, etc) and only then by their magnitude, i.e. sorting numbers of different types may not give the expected result.
merge(A, B, M) is equivalent to merge(0, =<, A, B, M).
Success:
merge([2,4,6],[1,3,5],L).
(gives L=[1,2,3,4,5,6]).
merge([f(1),f(7)],[f(8),f(10)],L).
(gives L=[f(1),f(7),f(8),f(10)]).
merge([f(5),f(8)],[f(1),f(2),f(2),f(5),f(8)],L).
(gives L=[f(1),f(2),f(2),f(5),f(5),f(8),f(8)]).
merge([a,w],[a,b,b,r,w],L).
(gives L=[a,a,b,b,r,w,w]).
merge([f(2),f(1)],[f(3),f(8)],L).
(gives L=[f(2),f(1),f(3),f(8)]).
Fail:
merge([2,4,6],[1,3,5],[1,2,3,4,5]).